1. Field of the Invention
This invention relates to an improved process for application of a metal-comprising coating to a metal-comprising surface. More particularly, a method is disclosed which enables application of the coating without use of a flux in contact with the metal-comprising surface. The coating is applied in an atmosphere which is inert at least with respect to the coating material during the time period of its application.
2. Background of the Invention
Soldering and brazing are well known methods of joining metallic items to each other. Both involve wetting metal parts with a liquid "filler" metal at elevated temperature and then allowing the parts to cool below the solidus of the filler metal. When the filler metal solidifies, the parts are joined.
Tinning and galvanizing are well known processes whereby parts comprising a metal are given a protective metal coating by dipping the parts into a molten bath of the metal which is to provide the protective coating.
Soldering, tinning, galvanizing and similar processes which involve the wetting of a solid, metal comprised surface with a liquid metal-comprising coating require that the surface and the coating be clean and free from compounds which prevent wetting during application of the coating. Normal handling of the metal-comprising parts and the metal-comprising coating often results in the formation o compounds which are detrimental to wetting, such as metal oxides. In addition, the environment of the coating application process itself frequently causes the formation of such compounds. Other compounds typically formed during processing which are harmful to good wetting include chlorides, sulfides, carbonates and other ionic, covalent and organic compounds.
To obtain good wetting, and thus good bonding of coating to surface, such compounds must be reduced back to a metal, reacted to form another compound which is not detrimental to wetting, or removed by techniques such as dissolution or mechanical cleaning.
U.S. Pat. No. 4,538,757 to Bertiger, dated Sept. 3, 1985, discloses a method of wave soldering in an atmosphere comprising a gaseous reducing agent (hydrogen). However, practical experience has indicated the hydrogen reacts with metal oxides etc. at temperatures greater than about 600.degree. C. Many of the plastics and resins comprising an electronic device such as a printed circuit board melt, degrade or change from their intended dimensions at the temperatures required to reduce oxides to a metallic state.
U.S. Pat. No. 4,606,493 to Christoph et al., dated Aug. 19, 1986, discloses a method and apparatus for soldering plate shaped circuit carriers whereby heated reducing gas (at about 600.degree. C.) is used to reduce oxides at the solder side of the carriers. The high temperature gas is only briefly impinged upon the solder side of the carrier so that other materials comprising the board are not affected by the exposure to the high temperature gas. However, it is impractical to apply individual jets to heat only the metal-comprising surfaces of the carrier, and the areas adjacent to the metal-comprising surfaces will exhibit the same temperature as the metal-comprising surface.
The compounds detrimental to wetting can be reacted or dissolved and washed away from the metal-comprising surface or from the surface of a metal-comprising coating bath using a fluxing agent. Fluxing agents are the most commonly used method of obtaining a wettable part surface at the time of application of the metal-comprising coating. However, fluxing agents are typically corrosive and residues must be removed from the part after coating. In addition, should the amount of flux be insufficient, the solder or coating material can form icicles on the part as the part is removed from the coating bath; wetting time may be inadequate within the process so that poor bonding between part and coating occurs; and, dross production during processing may be high, leading to heavy maintenance requirements.
A method of soldering without a flux is disclosed in Japanese patent application No. 9,756/77 of K. Noboku et al., filed Feb. 2, 1977. Noboku et al. bring the article to be soldered into contact with a molten solder bath to which ultrasonic vibration is applied. The ultrasonic vibration is disclosed as causing cavitation which breaks up the oxidized metal on the part to expose an active, solderable surface. The major disadvantage of ultrasonic soldering is the potential for damage to the structural integrity of the part exposed to the ultrasonic vibration. In addition, ultrasonic soldering is geometrically dependent. The cavitating effect cannot reach some part areas such as the inside of small, plated holes of printed circuit boards.
There is a need in many soldering or metal coating operations, and particularly within the electronics industry, for an application method which does not require the use of high processing temperatures which can damage the part, which does not require the use of a flux, and which does not require mechanical vibration which can harm part structural integrity.